Clean-Up Work

Wherever changes occur, it is also necessary to ditch some ballast. This affects libraries that have been removed, that have been repackaged, or that coexist in C and Python implementations. The popular Python idiom (Listing 4) of importing the fast C implementation of a module first and then falling back on the Python implementation if this fails is no longer necessary. Python does this automatically. More details are available on changes to the standard library [11].

All exceptions must derive from BaseException. This implies, in particular, that string exceptions are no longer supported. The exception object now has a new __traceback__ attribute, which contains the traceback of the exception. The approach to both calling and fielding exceptions with arguments has changed. Programmers can now throw exceptions with arguments using raise BaseException(args) and field them with except BaseException as variable (Figure 3).

Figure 3: The new traceback attribute for exceptions.

Python 3 also includes other changes to make life easier for programmers. For example, in cooperative super calls, it is no longer necessary to name the class instance and the class name. Old-style classes, which were deprecated at some previous time, no longer exist in Python 3.0; this removes the need to derive from object to use Python's newer features.

Direct evaluation of input via the input() command is no longer supported, as the input is available as an input string. This approach closes a critical security hole (Figure 4). It was only logical to rename the raw_input() function input() and to remove raw_input.

Figure 4: Direct evaluation of input via the input command is no longer supported in Python 3.

Of course, any description of the new features can't hope to be exclusive. If you want to know more, check out the reference document by von Rossum, "What's New In Python 3.0" [12].

Porting to Python 3.0

A clear migration path is available for porting Python 2 code to Python 3 (Figure 5), but you will need to test the code and fix any bugs at each step of the way.

Figure 5: The recommended migration path for porting Python 2.6 code to version 3.

The four lines of code in Listing 5 will serve as an example for migrating Python 2 to Python 3.0. All four lines defined functional components of Python. The first function calculates the sum of three numbers, 2, 3, and 4, by applying these arguments to the Lambda function. The reduce built-in successively reduces the list of all numbers from 1 to 10 by multiplying the results of the last multiplication with the next number in the sequence. The last two functions filter words, starting with filtering uppercase letters out of a string.

The code works on Python 2.6, and you only need to perform Steps 3 and 4 for the port. The source code for this example is stored in a file called port.py.

Calling the Python 2.6 interpreter with the -3 option (Figure 6) shows incompatibilities with version 3: Both the apply function and the reduce function are no longer built-ins in Python 3.0. The code is easily fixed (Listing 6), and the deprecation warnings then stop.

Figure 6: The Python 3 option in v.2.6 points to issues with the port.

The code generator 2to3 is really useful if you need to correct your Python 2 code; the generator's final step is to automatically generate code for versions 3.0 and 3.1. The tool offers several options for this (Figure 7). The direct approach is to overwrite the original file: 2to3 port.py -w. The result is the ported source code for Python 3.0 (Listing 7).

Figure 7: The 2to3 code generator has a large number of options.

When to Make the Move

Python 3.0 originally placed more emphasis on functionality, and this meant that it was about 10 percent slower than Python 2. The required optimization occurred in Python 3.1 [13]. This optimization relates to special handling of small integers. On top of this, Python 3.1's I/O library is implemented in C, which makes it between 2 and 20 times faster. Decoding of the UTF-8, UTF-16, and Latin-1 character sets is now twice to four times as fast.

If you are still waiting for third-party libraries to be ported, there is no point porting your application code to Python 3. von Rossum also recommends [14] not writing any code that will run on both Python 2.6 and Python 3 without modifications. It is preferable to maintain the source code as Python 2.6 code and then use automated tools to port to Python 3.0 or 3.1. Christopher Neugebauer has the final word in his video talk on Python 3000: "Learn 2.6, but keep 3k in mind."